The present invention relates to the microbial biotransformation of colchiconic compounds into derivative compounds, which are glycosylated exclusively at the C-3 position of the six-member ring. The process of the present invention provides the 3-O-glycosyl derivatives in high yields and purity.
A number of efforts using either chemical reactions or biotransformations have been made to obtain highly regiospecific glycosydated derivatives of compounds of formula (I), which is shown below, and related colchicinoid compounds.
For example, the chemical reaction route consists of sequences of complex, non-specific, non-selective reactions involving different molecular sites, which produce a mixture of glycosydated derivatives. Thus, the conversion yields of the desired effective or active product, which is specifically glycosydated at the C-3 position of the aromatic ring, are very low.
The biological approach substantially relates to the biotransformation of colchicinoid compounds such as colchicine and thiocolchicine, which are indirectly related with the colchicone compounds. For example, a known transformation, which is accomplished by a culture of Centella asiatica, yields derivatives, which are monoglycosydated at the C-2 and at C-3 positions of the aromatic ring (Solet, J. M., et al., Phytochemistry, 33, 4, 817-820, 1993). Thus, the transformation is not highly selective and also provides poor yields and productivity.
Other efforts to biotransform colchicinoid compounds have yielded simple demethylations of the methoxy groups bound to the aromatic ring at the C-2 and C-3 positions. These transformations are also characterized by limited yields, limited productivity, and by poor regioselectivity.
Attempts have been made to transform colchicine and its derivatives into the corresponding 3-demethylated derivatives using Streptomyces griseus and/or Streptomyces spectabilis (Hufford C. D. et al. J. Pharm. Sc., 68, 10, 1239-1242, 1979). Other workers have attempted the same biotransformation using different strains of Streptomyces and of other species of bacteria and fungi (Bellet P. et al. GB-923421, 1959). These results, however, confirm that these known microbial enzymes non-selectively produce the C-2, C-3, or C-10 derivatives of the alkaloid molecule. Moreover, the productivity of these catalytic systems are rather poor due to the low conversion yields, a requirement for reduced substrate concentrations, and frequent degradation of the tropolone ring.
More recently, Poulev et al. J. Ferment. Bioeng. 79, 1, 33-38, 1995 have obtained a specific demethylation using bacterial microorganisms, however, the demethylation also occurs with generally poor yields and productivity.
Enzymes from microorganisms similar to the above mentioned microorganisms, such as, for example, Streptomyces, Bacillus, have been applied to biotransform compounds, such as maytansinoids (U.S. Pat. No. 4,361,650 to Asai et al. and Izawa, M., et al., J. Antibiotics, 34, 12, 1587-1590, 1981). In these references, however, the catalyzed reaction consists exclusively of a demethylation characterized by low conversion yields and productivity.
Brumm, P. J., et al. (Starch, 43, 8, 319-323, 1991) have described the glycosyl transferase activity of an xcex1-amylase enzyme, which was derived from strains of Bacillus megaterium and has particularly high acceptor specificities for glucose or glucosides. For example, starting from starch, cyclodextrin-glucosyl transferases, produced by the same microbial source, catalyze the xcex1-1,4-transglucosylation of rubusoside (13-0-xcex2-D-glucosyl-steviol xcex2-D-glucosyl ester). It has been reported (Darise, M., et al., Agric. Bioel. Chem., 48, 10, 2483-2488, 1984), that, in this bioconversion, the acceptor of the transferase reaction is the substrate glucide fraction. Cyclodextrin-glycosyl transferases have also been used for preparing the cyclodextrins G6 and G8 from starch (Kitahata, S., Okada, S., Agric. Biol. Chem., 38, 12, 2413-2417, 1974).
These examples demonstrate the high substrate specificity toward glucosyl acceptors of glycosyl transferase enzymes expressed by Bacillus megaterium. Given this specificity, reactions toward substrates or metabolites having a different, complex molecular structure such as colchicones are entirely unexpected. In fact, no examples of the use of these microorganisms for the enzyme conversion of colchicone compounds to 3-glycosyl derivatives are known.
Now, it has been found that strains of Bacillus megaterium capable of growing in the presence of high concentrations of colchicone (R1=xe2x80x94OCH3, R2=xe2x80x94OCH3 in formula I below), 3-demethylcolchicone, or thio derivatives thereof, have exceedingly high, very specific activities for biotransforming such substrate compounds into derivative compounds, which are glycosydated exclusively at C-3 of the aromatic ring. The transformation takes place in very short times and is characterized by surprisingly high yields.
One aspect of the present invention relates to a process for the preparation of a compound of formula (I) 
which process comprises contacting a compound of formula (II) 
with Bacillus megaterium or a mutant thereof or an enzyme isolated from Bacillus megaterium or a mutant thereof, under conditions sufficient to effect a biotransformation of the formula II compound to the formula I compound, wherein R1 is a glycoside residue, R2 is C1-C6 alkoxy or C1-C6 thioalkyl, and R3 is OH or methoxy. The process further comprises recovering the compound of formula (I).
In a preferred embodiment, R1 is an 0-glucoside residue. Preferably, the compound of formula II is glycosylated exclusively at the C-3 position of aromatic ring A to obtain a 3-O-glycosylcolchicone compound.
In one embodiment, the process of the present invention comprises culturing the Bacillus megaterium in a medium comprising the compound of formula II in an amount sufficient to provide a recoverable amount of the compound of formula I, preferably from about 0.1 to 3 g/l. The medium may comprise water.
The Bacillus megaterium strain used in the process of the invention may be selected for the ability to grow in contact with the compound of formula II in an amount sufficient to produce recoverable amounts of the compound of formula I.
The medium may comprise at least one organic nitrogen source, which is preferably selected from the group consisting of meat extract, peptone, tryptone, casein hydrolysates, or corn-step water. In another embodiment, the medium comprises at least one carbon source, which is preferably selected from the group consisting of glucose, fructose, or glycerol. In yet another embodiment, the medium comprises at least one inorganic salt selected, which is preferably selected from the group consisting of K+, Na+, Mg++, or NH4+. The pH of the medium of the present invention is preferably from about 5 to 8, more preferably from about 6 to 7.
The culturing step is preferably carried out at a temperature ranging from about 20 to 45xc2x0 C., more preferably from about 28 to 40xc2x0 C. The culturing step is preferably, carried out at a maximum aeration level from about 1 to 2 liters of air per liter of culture per minute (vvm), more preferably from 1.5 to 2 vvm.
The compounds obtained by the biotechnological process of the invention, particularly thiocolchicosone (3-O-glucosylthiocolchicone, i.e., with reference to formula (I), R1=xe2x80x94OCH3e=xe2x80x94SCH3), are active principles of remarkable pharmacological importance, mainly for the preparation of new antitumor therapeutics.
Bacillus megaterium is a Gram-positive spore generating bacterium with a cell diameter greater than about 1.0 xcexcm. Bacillus megaterium is capable of growing aerobically on or within a number of culture media, is catalase-positive, and hydrolyzes gelatin.
Strains of Bacillus megaterium that are useful according to the invention grow satisfactorily and maintain their viability even in contact with high concentrations, for example, more than about 2 g/l, of colchicone and thiocolchicone substrates and their respective 3-demethyl derivatives such as, for example, wherein R1 is a glycoside residue, R2 is C1-C6 alkoxy or C1-C6 thioalkyl. Such ability may be evidenced, for example, by the examination of the growth and by microscope analysis. In contrast, congeneric species, such as Bacillus cereus, evidence difficulty in growing even at substrate concentrations of 1 g/l (absorbances of 10-15% of the control).
Considering the high yields of the present process, which range from about 70% to about 95%, the high selectivity and efficiency of the biotransformation is surprising and unusual.
Moreover, the microorganisms used in the bioconversion are capable of permanently maintaining catalytic activity, even after repeated fermentation steps. Thus, the microorganisms of the present invention may provide the regiospecific biotransformation in either fed-batch or continuous processes. Therefore, the present invention provides both high productivity and reproducible bioconversion levels.
The marked reaction regioselectivity of the present invention also assures that a desired product can be obtained in high quality, free from undesirable isomers. For example, with straightforward post-bioconversion processing, the product may be obtained in sufficient purity for use in, for example, therapeutic compositions. Preferably, the product is about 100% pure.
Further important advantages of the present process are the reduced complexity of the purification and recovery product steps, the low cost of the process, and the safety of the process.
Operative steps which are useful in the process of the present invention comprise are described generally below.
Initially, cultures of Bacillus megaterium capable of growing in the presence of high concentrations of a predetermined colchicone substrate are selected. Such cultures may be obtained, for example, from industrial samples or natural sources such as soil samples. Alternatively, cultures may be obtained from collection strains.
Isolates of the selected cultures are assayed for catalytic activity with respect to the biotransformation of the predetermined substrate into the corresponding 3-O-glycosyl derivative. In such a bioconversion assay, the substrate may be administered in gradually increasing concentrations. Additionally, the biotransformation yield, e.g., catalytic activity, of the selected strain toward the desired 3-O-glycosyl derivatives may be increased using a target specific selection procedure.
Parameters that enhance or optimize the biotransformation yield or catalytic activity of the selected strain or culture may be found and optimized. This step may comprise, for example, optimizing the fermentation conditions leading to optimal growth and/or conversion yields.
To provide stable homogeneous innocula for productive industrial scale applications, methods for conserving cultures having a desired catalytic activity are found and optimized.
Procedures and methods related to the above mentioned steps may then be scaled up for use in fermenter, batch, fed-batch and continuous processes.
Subsequent steps relate to determining and optimizing methods for post-biotransformation processing for the recovery of the transformed substrate.
At any point in the process, the selected strains may be characterized with respect to microbial properties as understood in the art.
Microorganisms usable in the present invention can be selected starting from collection cultures obtained from strain deposit centers, from soil samples of various origin, or preselected industrial strains. Selective recovery of the microorganisms may be accomplished on agar media comprising an organic nitrogen source such as, for example, peptones, yeast extracts, meat extracts, asparagine, or combination thereof, and a carbon source, such as, for example, glycerin, starch, maltose, glucose, or combination thereof. The media may have a pH from about 5 to 8, preferably from about 6 to 7. Incubation temperature range from about 20 to 45xc2x0 C., preferably about 28-40xc2x0 C.
As described above, colchiconic compounds may be toxic to microorganisms. The ability of a culture to grow in contact with or in the presence of a colchiconic substrate is evaluated by techniques such as serial dilution and plating the microorganisms in parallel on different agarized substrates, which comprise an amount of a colchiconic compound, e.g., 3-demethylthiocolchicone, which is sufficient to inhibit the growth of the majority of the microorganisms, preferably, the medium comprises from about 0.1 to 3 g/l of the colchiconic compound.
The colonies capable of growing in the presence of a substrate are withdrawn under sterile conditions and placed on different agarized media to verify the purity of the microorganisms and their homogeneity of growth.
Culture media used for conserving the culture, e.g., the microorganisms, are typical microbiological culture media. Such culture media may comprise organic nitrogen sources such as, for example, peptones, yeast extracts, tryptone, meat extracts, or combination thereof, a carbon source, such as, for example, glucose, maltose, glycerin, or combination thereof. Media further comprising other nitrogen sources or carbon sources suitable for growing or culturing microorganisms, as understood in the art, may be used in any step of the present invention. The media may also comprise other nutrients, such as ions or phosphorous, as suitable for microbial culture. The pH of the medium is from about 5 to 8, preferably about 6 to 7. The incubation temperatures range from about 20 to 45xc2x0 C., preferably from about 28 to 40xc2x0 C.
The selected microorganisms are then assayed for the capability of growing in submerged culture in the presence of colchiconic compounds and for the ability to transform the latter into the corresponding 3-glycosyl derivatives. Such assays were carried out in 100 ml flasks containing 20 ml of liquid medium, with different media formulations, comprising one or more organic nitrogen sources such as, for example, yeast extracts, peptones, tryptone, casein hydrolysates, meat extract, or corn-step liquor, one or more carbon sources, such as, for example, glucose, glycerol, starch, or saccharose, or sucrose, inorganic phosphorous and nitrogen sources, and inorganic salts of various ions, such as, for example, K+, Na+, Mg++, Ca++, Fe++, Mn++, etc.
Culture samples from each bioconversion assay, were analyzed to evaluate the production yield of 3-glycosyl derivatives, using thin layer chromatography (TLC) and high pressure liquid chromatography (HPLC).
The ability of the selected microorganism to transform colchiconic substrates into their corresponding 3-glycosyl derivatives was confirmed using bioconversion assays in flasks, on a 300 ml scale. The culture broths were the same as those used in the selection step.
Microorganisms that gave a positive response were used in tests for selecting conditions that enhance or optimize the bioconversion yield or catalytic activity. In particular, the optimized parameters included sources of carbon and organic nitrogen, mineral salts, temperature, stirring and aeration rates, pH, incubation time, innoculum ratio, subculture steps, and the time and form of addition of the substrate to be transformed.
The selected bacterial microorganisms, which are capable of effecting the biotransformation of the present invention, can grow on both solid and liquid culture media. The culture media may comprise one or more organic nitrogen sources, preferably, yeast extract, meat extract, peptone, tryptone, casein hydrolysates, corn-steep liquor or combination thereof. Carbon sources useful in the media comprise, glucose, fructose, saccharose, glycerol, malt extract, or combination thereof, preferably, glucose, fructose and glycerin. Additionally, glucose can be replaced by other sugars, such as, for example, fructose or galactose, without causing the loss of the glycosyl transferase activity.
The culture medium also comprises inorganic phosphorous sources, and ionic salts such as, for example, K+, Na+, Mg++, NH4+, or combination thereof.
The selected microorganisms can grow at temperatures from about 20 to 45xc2x0 C., preferably from about 28 to 40xc2x0 C. The microorganisms can also grow in a medium having a pH of from about 5 to 8, preferably from about pH 6 to 7.
Under the conditions described above, the selected microorganisms are capable of transforming the colchiconic compounds into the corresponding 3-glycosyl derivatives. The transformations occur, for example, in submerged culture, in flasks incubated on a rotating shaker, with stirring from 150 to 250 rpm.
Due to the particular kinetics of the present biotransformation that are related to the growth of the microorganisms, the optimum conditions for the biotransformation and microbial growth are the identical. Therefore, culture media and conditions useful to promote good microbial growth, such as those based on the organic and inorganic components cited above, are also useful for obtaining a good biotransformation activity of the colchiconic substrates into their corresponding 3-glycosyl derivatives.
The substrate may be added to the culture in the initial fermentation step, or the substrate may be added in fractional aliquots starting at the initiation of fermentation.
In order to obtain mutants having the desired biotransformation activity, any of the culture samples described above can, optionally, be subjected to mutagenic treatments, using conventional mutagenesis techniques such as, for example, irradiation with ultraviolet light or other techniques. The resulting cultures may then be assayed for mutants or variants having a desired biotransformation activity or catalytic activity, as described above.
The biotransformation of the invention is based on an enzyme conversion, which starts during the growth exponential phase and continues with a progression parallel to that of the microbial growth. Maximum levels of converting the substrate into the corresponding 3-glycosyl derivative are reached within the first 48-72 hours, depending on the addition time of the substrate. Such conversion levels are preferably up to about 95% or higher.
The regioselectivity of the biotransformation is absolute: no 2-glycosyl derivatives have ever been found in the culture samples. The resulting products are exclusively extracellular.
The substrate to be transformed can be added to the culture in any form suitable for introducing the substrate to the microorganisms therein. For example, the substrate may be added in a solution of acetone or alcohol, in alcohol-water mixtures or solutions, or in dioxane.
The biotransformation of the invention can be scaled up to fermenter level, keeping the culture conditions unchanged, as far as the culture medium, temperature and processing times are concerned. In order to obtain sufficient growth, adequate levels of stirring and aeration are important such as, for example, aeration levels of about 1 to 2 liters of air per liter of culture per minute (vvm), preferably from about 1.5 to 2 vvm.
After separating the biomass from the liquid fraction by centrifugation and recovery of the supernatant or by microfiltration and recovery of the permeate, the products of the bioconversion can be extracted from the culture broths. The culture can also be treated with alcohols, to obtain an optimum recovery of the product.
The purification and the recovery of the biotransformation products can be carried out using chromatographic techniques such as, for example, separation on absorption resins and elution with an alcohol, preferably with methanol. The methanol or hydromethanol solutions containing the product can be further purified by extraction with lipophilic organic solvents, preferably with methylene chloride. After further treatments with mixtures of alcohols and organic solvents, the product can be obtained in a pure state by crystallization from the resulting alcohol solutions.